However, @nacin said that it's not compatible with the WordPress license, and the original PHP function is supported by PHP 5.3+ only.

I saw that Ryan ​was working on some IDNA encoder and there are probably other license-compatible solutions, but I'm not sure how much of an overhead would it be to add a 200-400 LoC class for this use case. Thoughts?

Still there are few more related tickets to that problem, but can't decide how urgent or widespread that is.